1. Field of the Invention
The present invention relates to a polypropylene resin composition and its injection molded article. More particularly, the invention relates to a polypropylene resin composition characterized by being obtained by use of a small amount of organic peroxide, by exhibiting a high fluidity, and by being capable of yielding therefrom an injection molded article superior in heat distortion resistance, hardness and balance between rigidity and impact strength, to a method for its production, and to its injection molded article.
2. Description of the Related Art
Polypropylene resin compositions have heretofore been employed for the production of injection molded articles. For example, U.S. Pat. No. 4,626,565 discloses, as a method for producing a polypropylene resin composition which is superior in low-temperature impact resistance, fluidity and paintability and which is inexpensive, a method comprising heat treating, at a temperature of 170 to 280° C., a composition comprising a crystalline ethylene-propylene block copolymer, an ethylene-propylene copolymer rubber, inorganic filler and an organic peroxide wherein the amount of the organic peroxide is 0.001 to 0.5 part by weight per 100 parts by weight of the block copolymer and the copolymer rubber in total.
Japanese Patent Laid-Open No. 8-283487 discloses, as a polypropylene resin composition which is superior in low-temperature resistance while having an extremely high MFR (80 g/10 min or more) and which is superior in impact resistance and strength while having a high rigidity and a high heat resistance, a polypropylene resin composition containing a polypropylene resin having an MFR of 150 to 1000 g/10 min and a copolymer having a portion made up of ethylene and α-olefin wherein the composition has an MFR of 80 g/10 min. It discloses also that a peroxide or inorganic filler may also be blended if needed.
U.S. Pat. No. 6,087,443 discloses a thermoplastic resin composition improved in low-temperature impact resistance and thermal rigidity resistance, which is a thermoplastic resin composition obtained by mixing a propylene homopolymer and/or an ethylene-propylene block copolymer with a heat-treated material resulting from a dynamic heat treatment of a mixture of a propylene homopolymer and/or an ethylene-propylene block copolymer having a melt flow rate of 0.5 to 10 g/10 min and an olefinic copolymer in the presence of an organic peroxide and a crosslinking aid.
However, the polypropylene resin compositions and their production methods disclosed in the patent publications shown above need a reduction in the quantity of the organic peroxide to be used. In addition, there is a request for further improvements in fluidity of polypropylene resin compositions and in heat distortion resistance, hardness and balance between rigidity and impact strength of injection molded articles.